Prolactin (PRL) is an essential hormone for lactation that also has reproductive, somatotrophic and osmoregulatory functions. PRL secretion is inhibited by two distinct dopaminergic (DA) systems, the tuberoinfundibular (TIDA) with terminals in the stalk-median eminence (SME) and tuberohypophysial (THDA) with terminals in the posterior pituitary (PP). PRL and ovarian steroids affect both DA synthesis, by modulating tyrosine hydroxylase (TH), and DA release. Our laboratory has shown that the PP also contains a potent PRL- releasing factor (PRF), a small peptide which is distinct from known PRL secretagogues. The overall objective of this proposal is to clarify the role of DA, PRF and ovarian steroids in regulating PRL secretion. The first aim will be to determine the interplay between DA, PRF(s) and ovarian steroids in generating the PRL surge on proestrus. The second aim will be to investigate the relative importance of DA and PRF for regulating basal PRL secretion. The third aim will be to examine the rapid effects of estradiol (E2) on PRL and LH secretion. The fourth aim will be to study the regulation of DA synthesis and release from the TIDA and THDA systems in vitro. The fifth aim will be to determine the hormonal regulation of TH activity in the TIDA/THDA systems. Given the complexity of the control of PRL release, several experimental models will be used. In vivo models include the immature rat treated with exogenous gonadotropin with its proetrous-like PRL surge, the posterior lobectomized (LOBEX) rat, and the stalk-sectioned (SS) rat. In vitro models include a hypothalamo-PP explant in a compartmentalized chamber, and incubation and perifusion of hemi-pituitaries. Measurements will be made of PRL release, DA content, release, turnover rate and de novo synthesis, kinetics of DA receptors and of TH activity. These studies will extend our understanding of the functional relationship between the endocrine and nervous systems, and will contribute to the diagnosis and management of problems related to pituitary adenomas, lactation and fertility.